Forward Error Correction is a powerful tool that allows telecommunications systems to increase the transmission distance reach with a minimal increase of cost. State of the art FEC algorithms are able to tolerate input bit error rate (BER) values equal or higher than 10−2. Depending on the modulation format used, this may correspond to almost 10 dB improved signal-to-noise ratio tolerance. As an example, in a 1600 km optical fiber link with 20×80 km amplified spans and 0.22 dB/km fiber attenuation coefficient, 10 dB of optical signal-to-noise ratio (OSNR) variation corresponds to 900 km of additional link distance, boosting the total distance to 2500 km.
It is known to use different FECs in optical transmission to deal with different propagation conditions, for example, from L. Poti, G. Meloni, G. Berrettini, F. Fresi, M. Secondini, T. Foggi, G. Colavolpe, E. Forestieri, A. D'Errico, F. Cavaliere, R. Sabella, and G. Prati, “Casting 1 Tb/s DP-QPSK Communication into 200 GHz Bandwidth,” ECOC'12, Amsterdam, NL, 2012, paper P4.19.
The possibility to change the FEC type is known from G. Gho and J. M. Khan, “Rate-adaptive modulation and coding for optical fiber transmission systems”, J. Lightw. Tech., vol. 30, no. 12, pp. 1818-1828, June 2012.
Powerful FEC algorithms, able to work with low OSNR and high input BER values, can increase the outage probability if the propagation conditions are worse than expected or change during the system life. Numerous factors can affect the propagation conditions. If the OSNR is high, a powerful FEC may be used, with the disadvantage of a relatively low quantity of data payload that can be carried. The FEC selected needs to be sufficient for the OSNR, whilst maximising the payload.
In addition, the transmission over the optical fiber may suffer from non-linear effects.
Current systems are not able to distinguish what factors are primarily affecting the performance, and so a sub-optimal error correction strategy may be used.